1. Field of the Invention
The present invention relates to a rotation angle detecting device for detecting a rotation angle of a rotating object and a rotation angle control system equipped with the rotation angle detecting device.
2. Description of the Related Art
JP-62-95402-A discloses a rotation angle detecting device in which a change in a magnetic field formed by a permanent magnet is detected by a pair of Hall elements, whose analog output signals are converted into trigonometric functions to calculate a rotation angle.
In this rotation angle detecting device, if two Hall elements are positioned in such that the phase difference between the output signals thereof becomes a preset angle a from each other, such as 90 degrees as shown in FIG. 8, each of the output signals 400, 402 of the Hall elements outlines a sinusoidal curve as shown in FIG. 9 when a rotating object rotates. In other words, the output signal 400 is a sine curve, and the output signal 402 is a cosine curve.
In case where: the rotation angle of a rotating object is θ; and the voltages of the output signals 400, 402 are respectively V1 and V2; the rotation angle θ can be obtained by the following expression.θ=arctan (sin θ/cos θ)=arctan (V1/V2)  (1)
However, the phase difference between the output signals of the Hall elements may change due to a variation in the angle α between two Hall elements 302, a variation in distance between the Hall elements, a variation in position of the permanent magnet and/or a variation in the distribution of the magnetic field formed by the permanent magnet. Such a change in the phase difference may hinder accurate measurement of the rotation angle.